Electrolytic capacitors (e.g., tantalum capacitors) are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. For example, one type of capacitor that has been developed is a solid electrolytic capacitor that includes an anode (e.g., tantalum), a dielectric oxide film (e.g., tantalum pentoxide, Ta2O5) formed on the anode, a solid electrolyte layer (e.g., manganese dioxide, MnO2), and a cathode. Various other layers can also be applied to the solid electrolyte layer, such as graphite and silver dispersion layers successively applied to the manganese oxide layer prior to welding the anode and cathode lead terminals onto the capacitor.
The solid electrolyte layer is generally designed to electrically connect the dielectric film and the cathode, and thus, must have a certain conductivity. In addition, the solid electrolyte layer is also designed to inhibit short-circuiting of the capacitor that results from the presence of defects in the dielectric film. For example, upon exposure to heat generated by a short-circuit current, a manganese oxide layer can be converted to an insulator and thereby inhibit further short-circuiting.
Nevertheless, despite the benefits of using manganese oxide as the solid electrolytic layer, other materials have also been utilized. For instance, some electrolytic capacitors have utilized a conductive polymer layer (e.g., polypyrrole, polythiophene, polyaniline, polyacetylene, poly-p-phenylene, and the like) as the electrolytic layer. Examples of such capacitors are described in U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, et al.
For instance, Sakata, et al. ""862 describes forming a conductive polymer layer by polymerizing an aniline monomer on a dielectric oxide film using an oxidant. Sakata, et al. ""862 states, however, that because such conductive layers are thin, they become damaged by thermal stress generated upon mounting the capacitor, thereby increasing leakage current. Thus, Sakata, et al. ""862 also describes forming a first conductive polymer layer formed on the oxide layer and a second conductive polymer layer formed on the first conductive polymer layer.
Moreover, Sakata, et al. ""428 describes a capacitor having an electron donor organic compound layer covering the dielectric oxide film and a conductive polymer layer as the solid electrolytic layer. Sakata, et al. ""428 states that the electron donor layer can reduce normalized leakage current at higher temperatures when using a conductive polymer as the electrolytic layer. Examples of such electronic donor organic compounds are said to be fatty acids, aromatic carboxylic acids, anionic surface agents (carboxylate or sulfonate), phenol and derivatives thereof, silane coupling agents, titanium coupling agents, and aluminum coupling agents.
Nevertheless, despite the benefits obtained by utilizing a conductive polymer layer, various problems still remain with the capacitors formed therefrom. For instance, capacitors utilizing a conductive polymer layer still tend to short-circuit and have a relatively high equivalent series resistance (xe2x80x9cESRxe2x80x9d), which refers to the extent that a capacitor acts like a resistor when charging and discharging in an electronic circuit.
As such, a need currently exists for an improved electrolytic capacitor that inhibits short-circuiting and has decreased ESR.
In accordance With one embodiment of the present invention, a solid electrolytic capacitor is disclosed that comprises an anode that contains a valve-action metal (e.g., tantalum, niobium, and the like) and a dielectric film overlying the anode. The capacitor also comprises a protective coating overlying the dielectric film, wherein the protective coating contains a relatively insulative, resinous material. In some embodiments, the resinous material is selected from the group consisting of polyurethane, polystyrene, esters of unsaturated or saturated fatty acids, and combinations thereof. For example, in one embodiment, the resinous material can be a drying oil, such as olive oil, linseed oil, tung oil, castor oil, soybean oil, shellac, and derivatives thereof.
The capacitor also comprises a conductive polymer coating overlying the protective coating. For example, in some embodiments, the conductive polymer is selected from the group consisting of polypyrroles, polythiophenes, polyanilines, polyacetylenes, poly-p-phenylenes, and derivatives thereof.
As a result of the present invention, it has been discovered that a capacitor can be formed that has a relatively low leakage current, dissipation factor, and equivalents series resistance. For example, in some embodiments, the capacitor has a normalized leakage current of less than about 0.1 xcexcA/xcexcF*V, in some embodiments less than about 0.01 xcexcA/xcexcF*V, and in some embodiments, less than about 0.001 xcexcA/xcexcF*V, where xcexcA is the measured leakage current of the capacitor in microamps and xcexcF*V is the product of the capacitance and the rated voltage of the capacitor. In addition, the capacitor can also have a dissipation factor of less than about 10%, and in some embodiments, less than about 5%. Furthermore, the capacitor can have a equivalent series resistance of less than about 1000 milliohms, in some embodiments less than about 500 milliohms, and in some embodiments, less than about 100 milliohms.
In accordance with another embodiment of the present invention, a method for forming a solid electrolytic capacitor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; forming a protective coating on the dielectric film, the protective coating containing a relatively insulative, resinous material; and forming a conductive polymer coating. In some embodiments, for example, the protective coating is formed from a solution containing the relatively insulative, resinous material. If desired, the solution may further contain a non-aqueous solvent. During formation, one or more layers of the protective coating may be dried. For example, in some embodiments, one or more layers of the protective coating are dried at a temperature of from about 50xc2x0 C. to about 150xc2x0 C.
In accordance with yet another embodiment of the present invention, a method for forming a solid electrolytic capacitor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; applying a solution to the anodized anode that contains a conductive polymer catalyst and a relatively insulative, resinous material; and thereafter, applying a conductive monomer to the anodized anode, wherein the conductive monomer polymerizes to form a conductive polymer coating.
In accordance with another embodiment of the present invention, a method for forming a solid electrolytic capacitor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; applying a solution to the anodized anode that contains a conductive monomer and a relatively insulative, resinous material; and thereafter, applying a conductive monomer catalyst to the anodized anode, wherein the conductive monomer polymerizes to form a conductive polymer coating.
Further, in accordance with still another embodiment of the present invention, a method for forming a solid electrolytic capacitor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; applying a solution to the anodized anode that contains a conductive monomer, a conductive polymer catalyst, and a relatively insulative, resinous material, wherein the conductive monomer polymerizes to form a conductive polymer coating.
Other features and aspects of the present invention are set forth in greater detail below.